The total area above mean high water is about 17 hectares - roughly the size of 17 rugby pitches scattered across a kilometre of sea. The individual islets are connected at low tide and by a series of small stone footbridges; at high water they separate again. The Skerries Lighthouse, first lit in 1717 and now Trinity House's, stands on the highest point of the largest island. The waters between the Skerries and Carmel Head on the Anglesey mainland are some of the most dangerous on the British coast: the tidal stream runs at up to 6 knots, the reef extends well beyond the visible rocks, and shipwrecks from at least the 17th century through the SS Castilian in 1943 are scattered across the seabed. The Royal Navy still maintains an exclusion zone around the Castilian because her cargo of munitions is still live.

If the Skerries are dangerous to ships, they are extraordinarily good to seabirds. Arctic terns - those small, white, scissor-tailed marvels that migrate from the Arctic to the Antarctic and back every year - nest here in nationally important numbers. So do common terns. The much rarer roseate tern breeds here too in small but persistent numbers; in 2018 two roseate tern chicks fledged from the Skerries for the first time in a decade, a small but heavily celebrated event in the British seabird-conservation world. Atlantic puffins, black-legged kittiwakes, herring gulls and lesser black-backed gulls all breed here as well. The islands form part of the Ynys Feurig, Cemlyn Bay and The Skerries Special Protection Area, which BirdLife International also classes as an Important Bird Area; the same site has been designated a Site of Special Scientific Interest under domestic law. Terns interchange regularly between the three sites and form part of a larger Irish Sea population that also breeds at Rockabill, off the coast of Ireland.

From April to August each year, RSPB wardens take up residence in the old lighthouse keepers' cottages, which Trinity House made available after automation in 1987. Their work is unglamorous but consequential: counting nests, controlling the invasive tree mallow that would otherwise smother the colony, putting up nest boxes for roseate terns, intervening when great black-backed gulls take chicks. The wardens see things ordinary visitors never will. In July 2005 they spotted a sooty tern, a tropical species that essentially never appears in British waters - a record so unusual it brought a small wave of obsessive birders chartering boats out of Holyhead for a quick look. For most visitors, though, the islands remain a place glimpsed from a charter boat: low rocks white with bird droppings, terns dropping like darts into the sea, the lighthouse standing white against the granite, and beyond it the swell rolling in from the Irish Sea exactly as the Vikings would have seen it.